While the present invention can be effectively used in a plurality of paper or sheet-handling systems, it will be described for clarity as used in electrostatic marking systems, such as electrophotography. In an electrostatographic reproducing apparatus commonly used today, a photoconductive insulating member may be charged to a negative potential, thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a developing powder referred to in the art as toner. During development, the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive insulating area. This image may be subsequently transferred or marked onto a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image or marking, the copy paper may be removed from the system by a user or may be automatically forwarded to a finishing station where the copies may be collected, compiled and stapled and formed into books, pamphlets or other sets.
As above noted, there are many systems that transport paper or other sheet media after the media is marked or treated. These marking systems could include electrostatic marking systems, non-electrostatic marking systems and printers or any other system where paper or other flexible sheet media or receiving sheets are transported internally to an output device such as a finisher and compiler station or stations.
These electrostatic marking systems have finisher and compilers located at a site after the receiving sheets (paper) have been marked. The stacker tray assembly in these compilers usually comprises a stacker tray, controller sensors and height stack switches. Sheet stacker assemblies are well known in the art such as disclosed in Xerox U.S. Pat. Nos. 5,188,353; 5,261,655; 5,409,202; 5,476,256; 5,570,172; 5,842,695; 6,443,450 and 6,575,461. The disclosures of these Xerox patents are incorporated by reference into this disclosure.
Today, there is no reliable effective and inexpensive cut sheet stacking elevator systems that are capable of continuously measuring the position and direction of the elevator that supports the paper stack.
In some current finishing devices and feeders of printing systems, paper elevator position control generally involves stack height switches, corner sensors, and comb brackets with multiple transmissive sensors/algorithms to determine elevator position and direction.
These methods require an elevator to initialize (home) at some position which is usually at the top or bottom of travel. They measure position in the middle of travel by counting from the home position using stepper motor steps or sensor steps using a linear encoder. Often this process requires the elevator to travel to the bottom (or top) of its range to home, and then to move to the desired intermediate position during printer cycle up. This method takes a long time and several sensors are needed to identify elevator location (limited capability) and elevator motion. None of these designs allow for identifying stacker/elevator location and motion/direction in real time.
As an example, one system uses a comb bracket and 3 sensors to identify motion and upper and lower position only. Relatively expensive sensors are located on the elevator that detects transitions on a “comb bracket” located at the back of the frame.